This invention relates to pipelines lined with a thermoplastic liner.
Pipelines constructed of steel, iron, concrete or other such materials are susceptible to corrosion from both external and internal causes. Consequently, it is known in the art to line such pipelines with a corrosion-resistant thermoplastic liner (such as, but not limited to, polyethylene, nylon and other polyolefins) to inhibit internal corrosion or to reline existing lines that are deteriorating in areas where it would be extremely expensive to install new lines.
There are two types of liners, loose-fitting liners, where the liner itself carries the containment pressure of the fluid in the liner, and tight-fitting liners. The latter type of liners line pipes so that the liner is flush with and tightly engaged with the interior wall of the pipe. The liner therefore is physically supported by the pipe and so may depend on the strength of the pipe to carry containment pressure of the fluid in the pipeline. Tight-fitting liners are advantageous because they may be less expensive than loose-fitting liners, since they do not have to bear significant loads, and because they maintain the original pipe inner diameter to the greatest extent possible. Tight-fitting liners may also provide a double containment system since the inner liner may bridge seal any perforations or holes in the pipe resulting from corrosion or other means. Methods of lining pipes with tightly-fitting liners are known in the art. Some processes for providing tightly-fitting liners are disclosed in the following patents.
Canadian Patent 1,241,262 (Whyman and Kneller), discloses a method of installing a pipe liner of synthetic polymer in a pipe section whereby powered rollers are first used to radially deform the liner to within the elastic-plastic range of the liner. The liner in its deformed state is then inserted into a pipe section by the application of longitudinal or radial stress. The liner is then released from the stress so that it may expand radially into tight contact with the inside wall of the pipe section being lined.
U.S. Pat. No. 3,856,905 (Dawson) disclosed a process for lining a conduit with a plastic liner by radially compressing the liner at a temperature of about 100 degrees C to a diameter smaller than its original diameter without increasing the length of the liner, reducing the temperature of the liner to about room temperature, positioning the liner within the conduit, and heating the liner to a temperature of about 100 degrees C to cause radially expansion of the liner within the conduit. Longitudinal elongation of the liner is avoided by supporting the liner in a rigid frame, or, where the liner was passed through a swaging die, by operating sets of feed rollers and take up rollers at the same longitudinal speed.
U.S. Pat. No. 3,959,424 (Dawson) discloses a method of lining pipe with a thermoplastic liner which involves reducing the diameter of the liner, inserting the liner in a pipe and allowing the liner to expand by recovery within the plastic memory of the thermoplastic liner.
Canadian Patent 575,424 (Lontz et.al.) discloses lining metal pipes with fluorocarbon polymer liners by sintering a polymeric liner of larger diameter than the pipe, cooling the liner, stretching the liner to reduce its diameter to slightly less than that of the pipe, placing the liner inside the pipe, and releasing the stretching force on the liner, optionally raising its temperature at the same time.
U.S. Pat. No. 3,462,825 (Pope et.al.) discloses a process for lining a tubular member with a liner of fluorinated polymer by drawing the liner through a reduction die into the tubular member without application of heat, and releasing the liner, whereupon its expands into tight engagement with the inner wall of the member.
The problem with pipes lined with tight-fitting liners is that although the interior of the pipe is protected from corrosion by the liner, the exterior of the pipe is unprotected. If damage due to corrosion of the pipe is not repaired, the physical strength of the pipe may be seriously weakened to the point where it is unable to bear the pressure load of internal pressurized fluid within the lined pipe or of the pressure of the earth above the pipe if it is buried. The liner will therefore fail, since tight-fitting liners are generally unsuitable for bearing such loads for extended periods of time.
Failure of the liner will result in a significant replacement cost and may be catastrophic if the pipeline contains a substance that is hazardous to the environment.
Leak detection systems for dual-pipe or jacketed pipelines having an annular space between the inner and outer pipes or the inner pipe and outer jacket are known in the art. The systems are associated with the leakage into this space. Examples of such systems are disclosed in the following patents.
U.S. Pat. No. 4,667,505 (Sharp) disclosed a jacketed pipeline system whereby fluid leaked from the pipeline is contained within an annulus between the pipeline and the jacket. Leak detection in the annulus is also disclosed.
U.S. Pat. No. 4,644,780 (Jeter); U.S. Pat. No. 3,339,415 (Wild); U.S. Pat. No. 3,299,417 (Sibthrope); U.S. Pat. No. 3,184,958 (Eaton) and U.S. Pat. No. 4,450,711 (Claude) all disclose dual-containment systems having leak detections associated with the annulus between an inner and an outer pipe or sleeve.
None of the above-mentioned leak detection systems is suitable for use with tightly-lined pipe, because in a tightly-lined pipe there is no annulus between the pipe and the liner. Leakage in the liner may be teted by regularly pressure testing entire pipe-sections or by running a mobile internal pipe leakage detection device, otherwise known as a "smart pig", through the pipeline. However, these well-known methods of testing pipelines are expensive and therefore seldom used. Moreover, these methods do not adequately monitor the integrity of the outer pipe after the liner has been installed.